A laboratory fume hood is a ventilated enclosure where harmful materials can be handled safely. The hood captures contaminants and prevents them from escaping into the laboratory by using an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that inhalation of and contact with the contaminants are minimized. Access to the interior of the hood is through an opening which is closed with a sash which typically slides up and down to vary the opening into the hood.
The velocity of the air flow through the hood opening is called the face velocity. The more hazardous the material being handled, the higher the recommended face velocity, and guidelines have been established relating face velocity to toxicity. Typical face velocities for laboratory fume hoods are 60 to 150 feet per minute (fpm), depending upon the application.
When an operator is working in the hood, the sash is opened to allow free access to the materials inside. The sash may be opened partially or fully, depending on the operations to be performed in the hood. While fume hood and sash sizes vary, the opening provided by a fully opened sash is on the order of ten square feet. Thus the maximum air flow which the blower must provide is typically on the order of 600 to 1500 cubic feet per minute (cfm).
The sash is closed when the hood is not being used by an operator. It is common to store hazardous materials inside the hood when the hood is not in use, and a positive airflow must therefore be maintained to exhaust contaminants from such materials even when the hood is not in use and the sash is closed. As the hazard level of the materials being handled and the resulting minimum face velocity increases, maintaining a safe face velocity becomes more difficult.
An important consideration in the design of a fume hood system is the cost of running the system. There are three major areas of costs: the capital expenditure of installing the hood, the cost of power to operate the hood exhaust blower, and the cost of heating, cooling, and delivering the "make-up air," which replaces the air exhausted from the room by the fume hood. For a hood operating continuously with an opening of 10 square feet and a face velocity of 100 fpm, the cost of heating and cooling the make up air could, for example, run as high as fifteen hundred dollars per year in the northeastern U.S. Where chemical work is done, large numbers of fume hoods may be required. For example, the Massachusetts Institute of Technology has approximately 650 fume hoods, most of which are in operation 24 hours a day.
Capital or investment costs is an important factor in the design of fume hood systems. This relates to the capital cost of the supply and exhaust fans, duct work, boiler and chillers, and other equipment related to the movement and conditioning of the outside air brought into and exhausted from the building through the fume hoods. The size, capacity and cost of this equipment is integrally related to the peak capacity of air volume to be exhausted from the hoods. This total volume is in turn directly related to the face velocities of those hoods. For example, a 20% reduction in the face velocity for which the building hoods are designed, from 100 FPM to 80 FPM allows for a 20% reduction in the required capacity of the system air handling equipment.
Consequently, there are strong economic reasons for using the lowest face velocity which still produces acceptable fume hood capture and containment. Much research has been performed recently on the factors affecting this minimum acceptable face velocity. For example, with a fume hood having no equipment in the first 6" back from the sash, uniform face velocity distribution across the face of the hood, and no high cross drafts, the face velocity can be set to 60 FPM and excellent containment will occur. However, spillage will occur at 60 FPM if people walk past the hood, someone waves their arms near the opening or supply air diffusers blow air past the corners in front of the hood. All these disturbances create cross drafts and challenges to the fume hood containment which can pull fumes out of the hood. Increasing the face velocity to 100 or 125 FPM significantly reduces the spillage caused by these factors. Above 150 FPM, the air flow into the hood can become turbulent creating eddy currents and local low pressure areas which can also create spillage.
Because of the above factors, many laboratories operate their hoods at 100 to 125 FPM. Others allow the face velocities to drop to 70 to 80 FPM when the laboratories are unoccupied and operators are not near the hood where they might create crossdrafts from their motions. A very few companies operate their hoods at 60 FPM, but only with strict operating guidelines in order to prevent disturbance of the fume hood's containment.
In order to save energy and reduce the peak air capacity in laboratories, fume hood control systems are presently used that maintain a constant face velocity independent of the sash opening. Early versions of these systems operated by changing volume in a two or three step operation based on the sash height or the amount of sash opening. Much better and more recent systems provide continuous control of the air volume based on sash position and are referred to as variable air volume systems. An example of one of these systems is described in U.S. Pat. Nos. 4,528,898 and 4,706,553. These systems work well, but are dependent on the operator lowering the sash. When the operator does lower the sash, the exhaust, and typically also the room supply air volume, are reduced proportionately which generates the energy savings. If many hoods are used in a building with these controls, both the average and typical peak total air volumes will be reduced due to the diversity in the hood's operation. In other words, it is unlikely that all the hoods will be fully open at any one time. A problem for the building designer, however, is in estimating how much diversity will actually occur in the building. Consequently, many designers take a worst case view and don't size the buildings capacity below or much below the 100% capacity assumption of all the hoods full open at the same time. This is done because the designer is concerned that the users will not lower the sash when leaving the hood area. This is unfortunate because studies have shown that operators spend only a small fraction of their time in front of the hood.
In an attempt to bypass the operator problem of not closing sashes some fume hood manufacturers have introduced devices such as shown in U.S. Pat. No. 4,774,878 that detect the presence of the operator in front of the hood and raise the sash to some preset position. When the operator moves away from the hood, the sash is automatically closed. Typically, a two state or variable air volume control system is also used to vary the air volumes to maintain a constant face velocity at the two different sash positions.
These sash operator systems have not as of yet received widespread acceptance among researchers for several reasons. Firstly, the rapid movement of the sash up and down can occur even when a person just walks past the hood, producing a disturbing false reaction of the hood. Also, many researchers like to operate the sash at various heights, and this is made more difficult by the two position operators. Further, many hoods have wires, tubes and small hoses going into the hood near the bottom of the sash opening. Uncontrolled movement of the sash might hit these wires and hoses and potentially tip over delicate glassware to which the tubes and hoses are connected. This in turn could create a serious and potentially dangerous accident. Lastly, many hoods have horizontally moving sashes which make it difficult to implement a system to move the sashes in order to increase or decrease the amount of hood opening.
For all of the above reasons, a better approach is needed for reducing both energy usage and peak estimated replacement volume while not creating a potential hazard and not adversely affecting the researcher's work.